One other point is that if you use a unique constraint to function like a primary you MUST have the column defined as NOT NULL.

create table dbo.Car (

CarID int not null identity(1,1),

VIN varchar(25) unique

);

go

create unique clustered index CX_Car_CarID on dbo.Car

(CarID)

with (fillfactor=90);

go

insert Car

select null

select * from car

Notice I removed the NOT NULL constraint on VIN. You can't make that mistake if you define the column as the primary key. If you don't specify your column will allow NULL.

create table dbo.Car (

CarID int not null identity(1,1),

VIN varchar(25) unique

);

go

create unique clustered index CX_Car_CarID on dbo.Car

(CarID)

with (fillfactor=90);

go

insert Car

select null